improve token exchange sample curl

Author: amanda-ariyaratne

en/includes/guides/authentication/configure-token-exchange.md

@@ -107,6 +107,7 @@ Follow the steps given below.
     curl --location '{{ product_url_format }}/oauth2/token'
     --header 'Content-Type: application/x-www-form-urlencoded'
     --header 'Authorization: Basic <base64 Encoded (clientId:clientSecret)>'
+    --data-urlencode 'scope=<desired scopes>'
     --data-urlencode 'subject_token=<jwt_token>'
     --data-urlencode 'subject_token_type=urn:ietf:params:oauth:token-type:jwt'
     --data-urlencode 'requested_token_type=urn:ietf:params:oauth:token-type:access_token'